How to troll desperate shoppers

Well, not if he never worked there in the first place...

I mean, he can get people who actually work there in trouble

No, but it can get you arrested.
Impersonating an employee can be considered trespassing because you aren't a guest for shopping.

well, yes, it could, if a manager believed the story and blamed an employee, it could get someone in trouble and fired, since the manager might have no idea it was a fake employee